Hardenable stainless steel



nited States Patent STAINLESSSTEEL (Jar-1 S; Walton, Sewickley,andWilliamF. White, Whitehall Borough, Pa., assignors to: United StatesSteel Gorporatiom; a corporation of- New Jersey No Drawing.Applicationlanuary 4, 1955 Serial-No. 479,855

3 Claims. (Cl. 75-424) This invention relates. to improvements intheartof.

annealing. We have discovered. thattlie: addition ofnitrogen withincertain limits to steels of such type will prevent hardening followingsolution annealing and render the same uniformly responsive totransforming and precipitation hardening treatments.

The teachings of our invention are applicable to chromium-nickelstainless steels containing nickel in quantities insuflicient to renderthe same stably austenitic and which can be transformed and hardened bythermal treatment The preferred steel of our invention which has adesirable combination of corrosion resistance and mechanical propertiesand which at the same time will not transform following solutionannealing is uniformly responsive to transformation and aging treatmentshas the following preferred composition ranges:

with the remainder iron and residual amounts of other elements.

In the absence of nitrogen, such an alloy will at times respond totransformation and aging treatments. However, the response varies widelyfor reasons unknown and frequently the steel hardens to above 95 Rfollowing solution annealing and cooling. This has materially affectedthe usefulness of these steels since many heats had to be diverted toother uses. We have discovered that the addition of nitrogen in suitableamounts will prevent such hardening after solution annealing and willprovide a steel which will have hardness value of less than 95 R aftersolution annealing and air cooling. The nitrogen may be added by anyconventional method, such as by the addition of ferro-chromiumcontaining high nitrogen to the liquid steel melt. With our preferredanalysis containing between .12 and .18%-nitrogen, after solutionannealing, the steel can be treated either by heating and air cooling totransform it' or'by cooling;

to a low temperature. Heretofore, it has been necessary to vary thecomposition dependent on Whether itwas desired to transform the steel byheat treatment or'by' cooling to sub-zero temperatures. Followingtransformation our: steel can be aged to a hardness in excess of 38 RSteels having a composition within the foregoing ranges canbe suitablysolution annealed by heating to a temperature between 1800 and 2000 F;for sufficient time to permitthe carbidesto go into solution. Apreferred treatment is holding for 30 minutes at 1850 F. Following thisthey can-be air cooled to room temperature with substantially nohardening or transformation taking, place and will have a hardness valueless than R Since the steel'insuch conditionis quite soft and ductile,it can be readily formed and Worked. After forming or fabrication, thesteel can be transformed by either holding at a temperature below about50 F. for sufficient time to cause the austenite to transform to'martensite or by holding for sufficient time ata temperaturebetween 1300and 1500 F. to raise the M point sufficiently to permit the steel totransform on cooling to' room temperature. A preferred low temperaturetreatment is toholdforone hour at l10 F; and a preferred heat treatmentis to hold it for about two" hours at 1400 F;

The steel can then. be further hardened to avalue above 38 R by an agingtreatment which may vary considerably over a temperature range of 700 to1100 F. At the lower temperatures considerably longer times are requiredbut a suitable treatment is about 30 minutes at 900 F.

Hardness values following solution annealing, transforming and aging ofthe heats set forth in Table I will amply demonstrate the teachings ofour invention:

Table I Heat 0 Mn .P S s1 N1 01' Al N The hardness values followingsolution annealing, transforming and aging of the foregoing heats aregiven in the following Table II, wherein all hardness values areRockwell C unless otherwise specified.

Table II Transformation Transformation-l-Agiug Treatment Treatment 30Mln. at Heat 1,850 F.

No. Air 0001 1 Hr. at 2 Hrs. at

to RJI. 1 Hr. at 2 Hrs at F.+ 1,400 F.+ -1l0 F. 1,40 F 30 Min. at 30Min. at

1 Rockwell B.

Heat No. 1 containing .12% nitrogen was soft as an nealed and could behardened to a minimum hardness Patented June 30,, 1 959- value of 38 Rby both transformation treatments, where- I as Heat No. 2 containingonly 0.034% nitrogen exhibited a high hardness after annealing.

Heat No. 3 having an aluminum content below the preferred range did notharden upon aging to the specified minimum.

Heat No. 4 containing .15% nitrogen reacted much like Heat No. l.

Heat No. 5 with three elements slightly outside the prefelred range didnot transform and thus could not be hardened the desired amount.

Heat No. 6 with two elements outside the range on the side of high Mtemperature showed considerable transformation on annealing and thus wasnot satisfactory.

The steels of our invention are further characterized by an elongationof about 8% and a Charpy V notch impact value of at least 5 ft. lbs.when heat treated to a hardness value of R,, 41 and a correspondingtensile strength of 165,000 p.s.i.

While we have shown and described several specific embodiments of ourinvention, it will be understood that these embodiments are merely forthe purpose of illustration and description and that various other formsmay be devised Within the scope of our invention, as defined in theappended claims.

We claim:

1. A stainless steel characterized by a hardness value of less than 95 Rafter solution annealing and air cooling and susceptible to phasetransformation and agehardening by double thermal treatment to a.hardness value of at least R, 38 containing between .05 and .08% carbon,16.0 to 17.5% chromium, 5.50 to 6.75% nickel, .90 to 1.80% aluminum, .12to .18% nitrogen and the balance iron and other elements in amountswhich do not adversely affect the properties. 7

2. A stainless steel characterized by a hardness value of less than Rafter solution annealing and air cooling and susceptible to phasetransformation and agehardening by double thermal treatment to ahardness value of at least R 38, said steel containing between .05 and.08% carbon, 1.5% maximum manganese, 1.00% maximum silicon, 16.0 to17.5% chromium, 5.50 to 6.75% nickel, .90 to 1.80% aluminum, .12 to .18%nitrogen with the balance iron and residual amounts of other elements.

3. Age-hardened substantially martensitic stainless steel having ahardness in excess of 38 R characterized by initially having asubstantially austenitic structure following solution annealing and aircooling and a hardness in said condition of less than 98 R said steelhaving been transformed to a martensitic structure by thermal treatmentand thereafter age hardened, said steel containing between .05 and .08%carbon, 16.0 to 17.5% chromium, 5.50 to 6.75% nickel, .90 to 1.80%aluminum, .12 to .18% nitrogen and the balance iron and other elementsin amounts which do not adversely affect the properties.

References Cited in the file of this patent UNITED STATES PATENTS2,489,520 Camras Nov. 29, 1949 2,505,763 Goller May 2, 1950 2,703,298Branson et al. Mar. 1, 1955 OTHER REFERENCES Uhlig: Trans. A.S.M., vol.30," pages 949-980, especially pages 947, 954, 958, 978. 1942.

1. A STAINLESS STEEL CHARACTERIZED BY A HARDNESS VALUE OF LESS THAN 95RB AFTER SOLUTION AMEALING AND AIR COOLING AND SUSCEPTIBLE TO PHASETRANSFORMATION AND AGEHARDENING BY DOUBLE THERMAL TREATMENT TO AHARDNESS VALUE OF AT LEAST RC 38 CONTAINING BETWEEN 0.5 AND 0.8% CARBON,16.0 TO 17.5% CHROMIUM, 5.50 TO 6.75% NICKEL, .90 TO 1.80% ALUMINUM, .12TO .18E NITROGEN AND THE BALANCE IRON AND OTHER ELEMENTS IN AMOUNTSWHICH DO NOT ADVERSELY AFFECTED FROM THE PROPERTIES.